In the present application the term "float glass" means sheet glass produced by the float process on the surface of a bath of a molten metal such as molten tin.
The present invention relates to an improved method of chemically strengthening a glass article formed of float glass by using an ion exchange technique and to be strengthened glass article which is obtained by the improved strengthening method and which features high strength and very little warp and retains the excellent surface created on the surface of the molten metal bath.
Float glass has widely been used in buildings and vehicles by reason of its excellence in many respects such as flatness, smoothness of surfaces and uniformity of thickness. Furthermore, in recent years applications of float glass to electronic devices and particularly to display devices such as liquid crystal displays and plasma displays have been remarkably increasing. Concurrently there is a trend toward preference for employment of thin sheet glass, including float glass, not more than about 3 mm in thickness. As the thickness is reducing, strengthening of sheet glass is acquiring greater importance.
For strengthening thin sheet glass it is usual to use a chemically strengthening method represented by an ion exchange technique since it is difficult to strengthen thin sheet glass by thermal tempering. Conventional ion exchange strengthening methods are classified into two types, viz. low-temperature type ion exchange and high-temperature type ion exchange. In the low-temperature type method, the glass is contacted with a source of alkali metal ions relatively large in ionic radius, such as potassium ions, at a temperature below the transition temperature of the glass to cause exchange of the large alkali metal ions for relatively small alkali metal ions, such as sodium ions, in the glass surfaces. In the high-temperature type method, the glass is contacted with a source of alkali metal ions relatively small in ionic radius, such as lithium ions, at a temperature above the transition temperature of the glass to cause exchange of the small alkali metal ions for relatively large alkali metal ions in the glass surfaces. By either method compressive stresses are induced in the ion-exchanged surface layers of the glass.
In applying an ion exchange strengthening method to float glass, a problem is warp of the strengthened glass article and this problem becomes serious when the glass thickness is small. For example, in the case of a float glass disk about 1 mm in thickness and about 300 mm in diameter the amount of warp of the strengthened disk reaches 0.4-1.3 mm so that flatness of the disk is seriously marred. Though it is wished to use float glass as the substrate material for optical disks or laser disks, the wish has not been met yet by reason or failure to satisfy the flatness requirements of such substrates. For example, in a laser disk of the aforementioned thickness and diameter, the amount of warp or deviation from flatness must be less than 0.2 mm. The principal cause of such warping of float glass by ion exchange strengthening is presumed to be diffusion of tin, or an alternative metal, used as the molten metal in the float process into the glass surface which is in contact with the surface of the molten metal bath. That surface of float glass will be called the originally lower surface, and the opposite surface the originally upper surface. Usually float glass strengthened by ion exchange warps such that the originally upper surface becomes a convex surface.
So far, good measures to cope with the above described warp problem have not been found yet. Therefore, it is necessary to physically grind and polish the originally lower surface of float glass before making an ion exchange treatment where it is important to prevent the strengthened float glass from warping. For example, Japanese patent application primary publication No. 58-115043 (1983), which relates to precise annealing of float glass to be strengthened by ion exchange for the purpose of reducing residual strains, shows the necessity of sand grinding of the annealed glass surfaces before making a low-temperature type ion exchange treatment. However, the surface grinding and polishing treatments are troublesome and involve considerable cost since the depth of the tin-containing surface layer to be ground away reaches 10-20 .mu.m. Besides, the grinding treatment is liable to cause breaking of the glass or to produce certain defects in the glass, and this becomes serious in the cases of treating thin float glass. Furthermore, the grinding treatment results in vanishment of the excellent glass surface characteristic of float glass. Therefore, the surface grinding measure cannot be employed in chemically strengthening float glass to obtain precise articles such as laser disk substrates.
As a glass strengthening method of a different type, U.S. Pat. No. 3,453,095 proposes to introduce tin into the surface layers on both sides of float glass. According to the proposal, which is based on the recognition that tin diffused into the lower surface of float glass from the molten tin bath is in the form of stannous oxide, the upper surface of float glass is treated with stannous oxide vapor such that stannous oxide is present in both the lower and upper surface layers of the glass in nearly equal concentrations. After that the glass is heated under an oxidizing condition to oxidize stannous oxide in the glass to stannic oxide to thereby produce compressive stresses in the tin-containing surface layers. Float glass treated by this method will be fairly good in flatness. However, the compressive stresses produced in the glass surfaces by the conversion of stannous oxide to stannic oxide do not reach desirably high values, so that the treated glass is in many cases insufficient in breaking strength and is not worthy of the term, chemically strengthened glass.